In recent years, in mobile communication, various kinds of information such as images and data as well as speech are subjected to transmission. Following this trend, it is expected that demands further increase for high reliability and high speed transmission. However, when high speed transmission is performed in mobile communication, the influence of delayed waves by multipath is not negligible, and transmission performance degrades due to frequency selective fading.
Multicarrier communication such as the OFDM (Orthogonal Frequency Division Multiplexing) scheme has attracted attention as one of counter techniques for frequency selective fading. In multicarrier communication, data is transmitted using a plurality of subcarriers of transmission rates suppressed to such an extent that frequency selective fading does not occur. Particularly, with the OFDM scheme, the frequencies of a plurality of subcarriers where data is mapped are orthogonal to each other, so that the OFDM scheme has the highest frequency efficiency in multicarrier communication and implements multicarrier communication with relatively simple hardware configurations. Therefore, the OFDM scheme has attracted attention as a communication method to be employed in cellular scheme mobile communication, and has been studied in various ways. Further, according to the OFDM scheme, to prevent intersymbol interference (ISI), the tail end of each OFDM symbol is attached to the head of that OFDM symbol as a CP (Cyclic Prefix). By this means, on the receiving side, it is possible to prevent ISI as long as the delay time of delay waves stays within the time length of a CP (hereinafter “CP length”).
By the way, recently, studies on multicast communication are underway. Multicast communication is not one-to-one communication such as unicast communication, but is one-to-many communication. That is, in multicast communication, one radio communication base station apparatus (hereinafter simply “base station”) transmits the same data to a plurality of radio communication mobile station apparatuses (hereinafter simply “mobile stations”) at the same time. By this multicast communication, in the mobile communication system, for example, distribution services of music data and video image data and broadcast services such as television broadcast are realized. Further, services using multicast communication may have to be provided in a relatively wide communication area which cannot be covered by a single base station, and, in this case, multicast communication covers the entirety of the wide communication area by transmitting the same data from a plurality of base stations. That is, multicast data is the same between a plurality of cells. Thus, in multicast communication, the same multicast data is transmitted from a plurality of base stations at the same time, and, consequently, a mobile station nearby the cell boundary receives mixed multicast data comprised of multicast data from a plurality of base stations.
Here, in a mobile station located nearby the cell boundary, if the OFDM scheme is employed in multicast communication, when a plurality of the same OFDM symbols transmitted at the same time from a plurality of base stations are received within the CP length, these OFDM symbols are combined and received in a state their received power is amplified. To correct the channel fluctuation (phase fluctuation and amplitude fluctuation) of the combined signals by channel estimation, channel estimation values for these combined signals are necessary. Therefore, in multicast communication using the OFDM scheme, for a pilot used to calculate channel estimation values, the same pilot needs to be transmitted from a plurality of base stations at the same time, as in the case of multicast data. That is, multicast data pilots need to be common between a plurality of cells.
On the other hand, in unicast communication, a plurality of base stations transmit respective data (unicast data) (see Non-Patent Document 1). That is, unicast data differs between multiple cells. Here, unicast data refers to, for example, control data needed to transmit user data from a mobile station. By this means, for the pilot used to calculate channel estimation values in unicast communication, different pilots need to be transmitted from a plurality of base stations as in the case of unicast data. That is, unicast data pilots need to be different between multiple cells.
Further, studies are underway on the method of mapping pilots upon multiplexing and transmitting multicast data and unicast data (see Non-Patent Document 2). Further, while multicast communication employs a mode of communication in which information is transmitted only to specific mobile stations subscribing services such as news groups, broadcast communication employs a mode of communication in which information is transmitted to all mobile stations as in today's television broadcast and radio broadcast. However, the multicast communication is the same as the broadcast communication in that one base station transmits the same data to a plurality of mobile stations at the same time. Therefore, this document describes an MBMS (Multimedia Broadcast/Multicast Service) combining multicast and broadcast. Further, in this document, broadcast is used for explanation instead of multicast.
Non-Patent Document 1: 3GPP TSG RAN WG1 LTE Ad Hoc Meeting (2005.06) R1-050589 “Pilot channel and scrambling code in evolved UTRA downlink”
Non-Patent Document 2: 3GPP TSG RAN WG1 LTE Ad Hoc Meeting (2006.01) R1-060182 “MBMS Structure for Evolved UTRA”